Swimming pools are often equipped with filter units to remove particulate and gelatinous contaminants from swimming pool water. A typical filter unit includes a tank and an underdrain assembly located therein. The underdrain assembly is positioned at the bottom of the tank and is covered with a filtrate medium, such as sand. Dirty pool water is injected into the tank and onto the sand. As the water flows through the sand, it is filtered of its contaminants. The filtered water collects at the bottom of the tank. The underdrain assembly separates the filtered water from the filtrate medium prior to the return of the filtered water to the swimming pool.
A typical underdrain assembly is disclosed in U.S. Pat. No. 3,625,365 in which a plurality of laterals extend radially from a center collector hub. The laterals, which function to separate the filtered water from the filtrate medium, are rigidly snap-locked onto the center collector hub. Such an underdrain assembly is difficult to insert into a filter tank through the usual opening provided in the top thereof. First, the center collector hub must be inserted into the tank. Then, each lateral must be inserted into the tank and snap-locked onto the central collector hub. Since a person must place his arms through the opening in the top of the tank in order to assemble the underdrain, the assembly process is very cumbersome and time consuming. Alternately, a tank comprising upper and lower halves can be employed. The assembled underdrain is placed in the lower half of the tank, and sealed with the upper half. However, such a two-piece tank design is undesirable.
The prior art is replete with underdrain assemblies that overcome the assembly problems mentioned above. For example, U.S. Pat. Nos. 3,747,768 and 4,753,726 disclose underdrain assemblies having laterals that are pivotally mounted to a central collector hub. At one end of each lateral is a ball which fits into a corresponding socket located in the central hub. The laterals are folded up and inserted through an opening in the top of the tank. Once inside the tank, the laterals are folded downward into their operating positions.
Problems still exist with underdrain assemblies, such as those disclosed in U.S. Pat. Nos. 3,747,768 and 4,753,726, which employ pivotally-mounted laterals. For instance, if the laterals are not properly aligned, the performance of the filter is degraded. Once unfolded, the pivotally-mounted laterals of such underdrain assemblies are free to move, thereby creating the possibility of subsequent misalignment and hence ineffective filter operation. Furthermore, if a single lateral of such an underdrain assembly needs to be removed for cleaning or replacement purposes, the entire underdrain assembly must be withdrawn from the tank and the central hub must be disassembled. This procedure is both time consuming and burdensome.